1. Field of the Invention
The invention pertains to an improvement in head-up displays projected onto a safety glass laminate, wherein film between two glass layers.
2. Description of the Related Art
In use today in passenger vehicles are so-called head-up displays (HUD), which reflect information into the driver's field of view. To do so, an image is projected into the windshield from -below from a projector unit located in the dashboard, which can be viewed from the driver's seat as a virtual image. The effect is based on the reflection effect of the glass surface facing the interior of the vehicle, and on the glass surface that faces outward from the vehicle. The inner glass surfaces contribute almost nothing to the reflection since they are glued together by way of a PVB film located between the glass layers as is federally prescribed for composite safety glass, the PVB film having a refractive index similar to glass.
The following convention, shown in FIG. 1, is used to simplify the naming of the four glass surfaces of a typical windshield design:    Side 1 The outer surface of the composite member pane facing the outside of the vehicle    Side 2 The inner surface of the outer pane in the composite member facing the interlayer film    Side 3 The inner surface of the inner pane in the composite member facing the interlayer film    Side 4 The surface of the inner pane of the composite member facing the interior of the vehicle
In general, a plasticizer-containing film of polyvinyl butyral, abbreviated as “PVB film,”, is used as the interlayer film.
To eliminate double images in a conventional HUD utilizing the functional principle described, the glass surfaces of sides 1 and 4 must be set at a specific angle relative to one another. This is accomplished through the use of PVB films having a wedge-shaped thickness profile. However, such films are difficult to process and complicated in their manufacture, which makes them much more expensive than non-wedge-shaped film counterparts.
Other disadvantages of conventional HUDs are that the information can only be visualized within a limited field of vision directly in front of the driver, and is only visible to the driver. However, there are in fact many situations in which information should be made visible to passengers or projected onto other locations of the windshield. For example, it is conceivable that the contour of obstructions on or next to the road, which are already detected in current vehicles by night vision systems, can be projected directly into the windshield to provide safety-related warning information.
It is also true in conventional HUD's that the image is blurred by raindrops located on the windshield since the refractive index transition between the glass and the air, which is the basis of the optimized system configuration, is compromised.
As an alternative to these conventional HUDs, it has already been proposed that images be generated using fluorescing materials disposed in the plane of the windshield, through excitation of the same so-called “fluorescence HUD”. The excitation of these materials is done using UV radiation which is invisible to the human eye, which generates real images in the plane of the windshield that are visible to more than just the driver. Suitable organic dyes, inorganic particles, etc., have been proposed as fluorescing materials.
A known method for manufacturing fluorescence HUDs involves applying fluorescing pigments or dyes in the form of coatings, laminated films or the like onto side 4 of the windshield. An alternative proposal was to position, between side 3 and the PVB interlayer, films containing fluorophores or to print the surface of the PVB film facing side 3 with fluorophores.
WO 2012/072950A1 discloses the printing of a PVB film with certain low molecular weight fluorophores in such a way that they are distributed evenly in the film during the autoclaving process. In this case, the fluorophores also act as UV absorbers. To this end, WO 2012/072950A1 describes the use of low molecular weight hydroxyterephthalates, in particular 2,5-dihydroxydiethyl terephthalate, as a fluorophore, and the use of antioxidants, which are applied to a PVB film. In the process, the fluorophore is distributed in the PVB film and can act as a UV absorber by itself or can mix with an existing low molecular weight UV absorber in the film.
WO2008132368 A9 discloses that the fluorescence HUD can contain one or more layers comprising inorganic luminophores in a thickness of less than 20 um on side 3 of a composite member in addition to a conventional PVB film. However, since conventional PVB films contain low molecular weight UV absorbers in small amounts, such a low molecular weight UV absorber can easily migrate into the layer containing the luminophores and extinguish the fluorescence there. There is also the risk that uncontrolled excitation can be caused by outside sunlight to some degree since conventional PVB films usually exhibit residual transmission for UV radiation. Care must also be taken to ensure that no light scattering occurs due to unsuitably large particles, which can lead to distortion. The small layer thickness of the layers containing the luminophores, which is 20 um, limit the useful amount of fluorophores since distortion occurs when the concentrations are too high. This limits the maximum achievable intensity of the fluorescence emission.
US 2002/0120916 A1 describes head-up displays comprising fluorescent dyes in which the side facing the observer contains UV absorbers. This is intended to prevent excitation radiation for the fluorescent dyes from radiating through the display onto the outer side. To this end, US 2011 1073773 A1 discloses fluorescent displays that comprise an opaque material on the side of the display facing away from the observer.
EP 2409833 describes a fluorescent display in which the fluorescent dyes are disposed on the side facing the sunlight.
These known fluorescent displays have the disadvantage that the fluorescent dyes can be excited by sunlight, which is undesirable. Such an excitation visually results in a distortion of the display, which is unacceptable for use in vehicle windshields.
According to the known prior art, the following problems occur which negatively affect the brilliance and sharpness of the information shown in the display as well as the persistence thereof. PVB films commonly available for the manufacture of automobile glass contain small amounts of low molecular weight UV absorbers, in particular of the Tinuvin 326 and Tinuvin 327 types.
Since the UV absorbers act in precisely the same wavelength range in which the excitation of the fluorophores is to take place, a large portion of the UV radiation is absorbed instead of being available for excitation of the fluorophores.
Therefore, when a low molecular weight fluorophore is simply applied to the surface of a PVB film that already contains a UV absorber, the fluorophore will distribute throughout the entire thickness of the film through diffusion. The fluorophore can then only emit in the direct vicinity of the glass surface since this is the only place where UV radiation impacts the fluorophore in sufficient intensity since it cannot significantly penetrate into the film layer due to the existing UV absorbers. This necessitates high concentrations of fluorophores in order to achieve a sufficient brilliance, which in turn can have a negative effect on the yellow tint of the film or glazing and on the costs of the display system. Of course, the same applies when the fluorophore is added to the PVB film directly as an additive during manufacture of the film.
It is especially complicating that fluorophores migrate as well to side 2 of the windshield (see FIG. 1) by way of diffusion, where on the one hand they can bleach out due to sunlight since there is no more protection provided at this point by UV absorbers contained in the film, and on the other hand they can be uncontrollably excited to a state of fluorescence by the UV fraction of the solar radiation.